This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        BS base station        BW bandwidth        CM cubic metric        CoMP coordinated multi-point        CQI channel quality indication        CRS common reference signal        CS cyclic shift        CSI channel state information        DL downlink (eNB towards UE)        DM RS demodulation reference signal (also referred to as DMRS)        eNB E-UTRAN Node B (evolved Node B)        E-UTRAN evolved UTRAN (LTE)        FDM frequency division multiplexing        FDMA frequency division multiple access        IFDM interleaved frequency-division multiplexing        LTE long term evolution of UTRAN (E-UTRAN)        LTE-A LTE advanced        MIMO multiple input multiple output        MU-MIMO multi-user multiple input multiple output        Node B base station        OFDM orthogonal frequency division multiplexing        OFDMA orthogonal frequency division multiple access        PDCCH packet downlink control channel        PRB physical resource block        PUCCH physical uplink control channel        PUSCH physical uplink shared channel        RAN1 technical specification group radio access network working group 1        RB resource block        Rel release        RPF repetition factor        RRC radio resource control        RRH remote radio head        RRM radio resource management        RS reference signal        SC FDMA single carrier, frequency division multiple access        SRS sounding reference signal        SU MIMO single-user MIMO        TDM time division multiplexing        UE user equipment, such as a mobile station, mobile node or mobile terminal        UL uplink (UE towards eNB)        UMTS universal mobile telecommunications system        UTRAN universal terrestrial radio access network        ZC Zadoff-Chu        
Modern wireless systems such as cellular systems use a number of reference signals (RSs), e.g., for data demodulation and channel sounding. Two important signals used in uplink are demodulation reference signals (DM RS or DMRS) and sounding reference signals (SRS). DM RS are primarily used for channel estimation for coherent demodulation, while the SRS are primarily used to channel quality determination, e.g., to enable frequency-selective scheduling. See Sesia et al., “LTE—The UMTS Long Term Evolution: From Theory to Practice”, ch. 16, “Uplink Reference Signals”, pp. 359-375 (2009), which also explains these signals in much more detail.
In particular, SRS is an important signal, allowing for link adaptation and frequency domain packet scheduling in the uplink, uplink timing estimation as well as precoder selection related to SU-MIMO. Furthermore, due to channel reciprocity in TDD, SRS can be utilized for DL link adaptation and precoding as well. The importance of SRS is further increasing in both UL and DL sides due to introduction of Coordinated Multi-point (CoMP) in LTE Rel-11.
With regard to CoMP, CoMP transmission is currently being investigated in 3GPP RAN1. The motivation for CoMP is to allow fast coordination among different transmission points to improve coverage of high data rate, cell-edge throughput, and/or to increase system throughput. To enable closed-loop transmission from multiple transmission points to a given UE, CSI for multiple radio links is measured by the UE and sent to the network using an uplink control channel (PUCCH) or an uplink data channel (PUSCH).
A UE in a CoMP scenario may be attached to a serving eNB and may communicate with that eNB for UL control (PUCCH), uplink data (PUSCH), and/or DL control (PDCCH) channels. For CoMP transmission, the UE can receive joint transmissions (PDSCH) from the serving eNB and/or one or more non-serving eNBs (e.g., from overlapping cells).
Uplink CoMP reception implies reception of the UE's transmitted signals at multiple geographically separated or co-located points (e.g., a single UE transmitting to multiple eNBs). In the DL direction where the eNB transmits data to the UE, DL CoMP transmission implies dynamic coordination among multiple geographically separated transmission points. Examples of DL CoMP schemes include coordinated beamforming where the data to a single UE is instantaneously transmitted from one of the transmission points and the scheduling decisions are coordinated to control, for example, the interference generated in a set of coordinated cells. In coordinated scheduling and coordinated beamforming, the data may only be available at a serving eNB and transmission scheduling may be coordinated among eNBs within the CoMP cooperating set.
It can be seen that RSs such as SRS serve important purposes in situations such as CoMP. For instance, in uplink, signals from one UE are now being transmitted to many eNBs (e.g., as opposed to a previously conventional UE-to-single-eNB communication). Current SRSs may not provide the flexibility or number of RSs available in these and similar situations.